Today's schools emphasize the use of student data to make instructional decisions. Standardized tests determine funding and teacher advancement (Datnow, Park, & Wholstetter, 2007; Gullo, 2013; Marchant & Paulson, 2009; Schraw, 2010). To evaluate learning before these tests, teachers must utilize their own assessments and data. Formative assessments serve as intermediary checks of learning that provide both teachers and students with information about learning progress (Heritage, 2007; Gullo, 2013). Systematic gathering, analysis, and planning based on this assessment data is termed data-driven instruction (DDI; Mandinach, 2012). Many Indiana school districts have adopted teacher evaluations emphasizing use of DDI (Maier, 2012). Therefore, the current study explores factors that influence Indiana teachers' use of DDI based on formative assessments. The Theory of Planned Behavior (TPB) is used as a theoretical framework to predict intent to conduct DDI and DDI frequency from teachers' attitudes, social norms, and perceived behavioral control (Fishbein & Aijzen, 2010). This study utilizes an exploratory mixed methods design to apply TPB (Creswell & Clark, 2010). Nine Indiana elementary teachers participated in focus groups exploring their DDI practices. This qualitative data was used to design a survey to collect quantitative data from a broader sample. Elementary teachers were recruited to participate via email and social media sites for professional education organizations to take the online survey. 283 current elementary teachers in Indiana completed the survey. Hierarchical linear regressions were used to test the hypotheses that TPB could explain teachers' intent to conduct DDI based on formative assessments and the frequency with which they do so. The hypotheses were partially supported, with TPB significantly predicting both intent (positively predicted by attitudes, social norms and measurement training and negatively by school SES) and behavioral engagement (positively predicted by intent). In both phases of the study, social norms emerged as a highly influential factor on teachers' DDI practices. The results of this study will be used to design targeted interventions for Indiana teachers on DDI. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]